This invention relates to a process and equipment for determining the alcoholic strength (alcohol concentration by volume) of an alcoholic beverage and water/alcohol solutions in general.
Various parties have an interest in determining the alcoholic strength of water/alcohol solutions.
Manufacturers of such beverages contract for the price of the materials from which they obtain distillates on the basis of alcoholic strength, for example the alcoholic strength of the wine used to produce brandy or the beer used to produce whisky. In addition to this, alcoholic strength is an important parameter in the process in distilleries, as the thermodynamic equilibria in the distillation columns are linked to it.
Also final consumers have a right to know the percentage of alcohol present in the drinks which they are purchasing.
Historically the first instrument for the measurement of alcoholic strength in a water/alcohol solution was Baumé's areometer, which was conceived in 1770. Two centuries later Baumé's areometer is still the most widely used instrument, particularly for measuring the alcoholic strength of distillates.
Baumé's areometer is nothing other than a density meter, in that the principle of its operation is based on the density of the water/alcohol solution being analysed. Water and ethyl alcohol have different densities, specifically 0.998 kg/l for water and 0.789 kg/l for alcohol at 20° C., and their mixtures therefore have densities lying between these two values.
Areometers are direct reading devices and comprise a graduated tube, generally of glass, ballasted in such a way that it adopts a vertical position of stable equilibrium when immersed in a liquid. The hydrostatic thrust (Archimedes thrust) from the water/alcohol mixture against the instrument determines the point at which the graduated tube emerges, directly indicating the alcoholic strength of the solution. This thrust is proportional to the volume of liquid occupied by the immersed body, multiplied by the density of the liquid itself. In order to obtain a direct reading the instrument must be calibrated against a graph relating density with alcohol concentration which (in the case of ethyl alcohol) has the shape shown in FIG. 1 of the appended drawings. As will be seen in that figure, the change in the density of a water/alcohol solution is one which regularly decreases, but is not linear in relation to concentration.
This constitutes a limitation for areometers. In order to overcome these problems precision areometers have a graduated scale restricted to between two not too distant values of alcoholic strength. In order to be able to make measurements throughout the range of values it is therefore necessary to have a set of areometers, each of which is intended for a particular specific range of alcoholic strength. Furthermore, as is the case with almost all substances, density changes in relation to temperature. In the- case of areometers this parameter constitutes a third variable which has to be taken into account. Normally areometers are calibrated at 20° C. and use must be made of adjustment tables, generally provided by the same manufacturer, for all temperature values differing from that reference value.
One form of development of areometers is represented by hydrostatic balance devices. These also determine the density of the liquid by measuring the hydrostatic thrust which the liquid exerts on an immersed glass bulb. This bulb, which contains a capillary thermometer within it, is hooked onto a precision balance through a metal wire. The density of the liquid can be obtained by reading the value of the hydrostatic thrust from the balance, allowing for temperature through suitable conversion tables.
Hydrostatic balance devices are very much more accurate instruments than areometers, but they are also more cumbersome and costly. They require a very stable supporting bench which is not subject to vibration of any kind. These are therefore items of equipment which are intended almost exclusively for laboratory use.
A further instrument which can be used to determine the alcoholic strength of a solution is a pycnometer, which comprises a small glass ampoule provided with a neck and a ground cap with a capillary hole. Using this instrument it is possible to take precisely known and identical volumes of distilled water and the solution whose density is to be determined. Through simple weighings carried out at a particular constant temperature using a precision analytical balance the weight of the empty pycnometer, the weight of the distilled water and the weight of the pycnometer with the solution under test are determined. The relative density of the solution under investigation, that is the density in relation to that of water, can then be calculated using a simple formula.
Around 1850, approximately one century after the invention of the Baumé areometer, another instrument to determine the alcoholic strength of a solution appeared—the Malligand ebulliometer or ebullioscope. This instrument makes use of the property that water/alcohol mixtures have different boiling points depending on the quantity of alcohol which they contain.
The Malligand ebulliometer comprises a metal boiler connected beneath to an annular tube which is inclined and welded to a small chimney beneath which a heating lamp is positioned. This annular tube makes it possible for the liquid present in the boiler to be heated by thermal siphoning. The boiler is closed with a screwed lid provided with holes and has a metal arm bent into a right angle. A thermometer passes through the central hole in the lid and its bulb dips into the boiler while its capillary, which is also bent into a right angle, is housed horizontally in a metal arm, against a graduated scale. A cooling unit, whose function is to cause condensation of the alcoholic vapours to prevent any change in concentration of the solution altering the boiling point, is housed in the side hole of the cover.
When making measurements the ebulliometer is filled with the solution under investigation, the lid is screwed on and the cooling device filled with water is fitted. Heating then begins and the maximum temperature reached is read off. This coincides with a graduation on the alcoholometric scale, which provides the alcoholic strength directly.
Alcoholic beverages obtained by distillation, like brandies, can be regarded as pure water/alcohol solutions from the physical point of view, that is they comprise a mixture of water and ethyl alcohol. All the other components, although important from the organoleptic point of view, are present in quantities which are too small for them to have an influence on the measurement of alcoholic strength.
In the case of drinks with a low alcohol content, like wines, the presence of other components like tannins, acids, etc., may result in incorrect measurement of the alcoholic strength when densitometric methods are used. In such circumstances the Malligand ebulliometer offers considerable advantages. In fact this instrument measures the boiling point of the solution under investigation and this measurement is more accurate the lower the alcoholic strength. FIG. 2 in the appended drawings shows the binary phase diagram for water/ethanol. The lower curve A, also known as the liquidus curve, represents the boiling points of the mixture; the upper curve B instead shows the composition of the vapour in equilibrium with the boiling liquid. It will be noted that boiling curve A has a much greater slope for low alcohol concentrations. For this reason the Malligand ebulliometer provides better accuracy up to alcohol concentrations of 20%.
Measurements made using a Malligand ebulliometer are approximate, but have the advantage of being quick to obtain. This instrument cannot however be used with solutions containing sugars, like liqueur wines, or carbon dioxide, like sparkling wines, because these substances have an appreciable effect on the boiling point. In these cases it is first necessary to perform a distillation of the beverage to purify it from the undesired substances and then measure the alcoholic strength.